The present invention relates to a process for precipitating elastomer-filler mixtures optionally containing extender oils.
As is known, the usefulness and processability of raw elastomer mixtures can be substantially influenced by the addition of fillers. Thereby, the properties of the final vulcanizates can be adapted to the requirements of the end use. Depending on the effect of the filler additives on the raw mixture and on the mechanical properties of the vulcanized products, the fillers are frequently classified as highly active, active, semiactive, and inactive. Highly active and active fillers are also called reinforcing fillers.
Besides the conventional carbon blacks, light-colored fillers are of industrial importance, primarily those based on silicic acid. While carbon black inherently exhibits a certain affinity to organic types of elastomers, this is not so with the light-colored fillers, especially light-colored fillers based on silicic acid. Considerable problems are thus encountered in admixing such light-colored fillers and, above all, in uniformly distributing them in the elastomer. This is true particularly for the processes wherein the light-colored reinforcing filler is incorporated into the elastomer using a conventional mixing unit in the rubber-processing industry (e.g., a rolling mill, masticator, etc.).
To increase the affinity between such fillers and the elastomer, and thus to shorten the requisite mixing periods, if possible, polar organic compounds are frequently employed with the fillers. These polar organic compounds, due to their hydrophilic portions, can be adsorbed on the fillers and thus render the filler particles more lyophilic. Materials of this type include, for example, stearic acid and benzoic acid. In spite of these auxiliary agents, the mixing procedures on the rolling mill or in the masticators still involve the danger that the elastomer component will be irreversibly damaged by the relatively long period of mixing still required.
Accordingly, many attempts have been made to develop processes for the production of elastomer-filler mixtures which are gentler to the elastomer component, primarily for the production of elastomer-filler mixtures which contain light-colored reinforcing fillers and specifically silicic acid.
Thus, a number of processes is known wherein the elastomer-silicic acid mixture is prepared by the combined precipitation of elastomer dispersions and alkali metal silicate solutions. The starting materials are emulsifier-containing mixtures of elastomer dispersions and alkali metal silicate. Precipitation is carried out while maintaining specific conditions by adding acids in the presence or absence of electrolytes. (See German Pat. No. 1,148,067 (U.S. Pat. No. 3,849,363); German Pat. No. 1,184,491 (U.S. Pat. No. 3,392,140); German Pat. No. 1,204,405 (U.S. Pat. No. 3,849,363) and addition to German Pat. No. 1,148,067; German Pat. No. 1,215,918; and German Pat. No. 1,297,858 (U.S. Pat. No. 3,190,851). The disadvantages of these processes include, on the one hand, the fact that the silicic acid obtained during the combined precipitation step cannot be adequately controlled with respect to its structure within the elastomer so that a constancy of properties exhibited by the thusformed elastomer-silicic acid mixtures cannot be satisfactorily maintained; and, on the other hand, there is a relatively large loss of unbound silicic acid, which is obtained in the residual fluid of the elastomer latex and cannot be reused.
It is furthermore known to produce elastomer-silicic acid mixtures by mixing elastomer dispersions with silicic acid dispersions in the presence of quaternary ammonium salts having one hydrogen atom. The elastomer-silicic acid mixtures are precipitated from the mixtures by the addition of acids and/or salts (See German Pat. No. 1,224,481 (British Pat. No. 1,030,616). This process, although enabling the incorporation of a silicic acid of a defined structure into the elastomer, still exhibits the drawbacks that an inordinately large amount of the silicic acid is not absorbed by the elastomer. Thus, either an expensive recovery procedure is required or the unused silicic acid constitutes a pollutant in the wastewater.